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Originally Posted by AP555
I was wondering if BMW should have used a taller 7th gear (DCT) instead of the start stop feature for better fuel economy.
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The two aren't mutually exclusive, of course, so it's unlikely they'd trade one for another.
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Higher RPM's = Higher fuel consumption
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Yes and no. There's an optimal RPM that trades consumption at a steady speed vs. consumption while accelerating or while under load changes. We could look at the extreme as an example - if you put in a gear to let the car go 70 mph at idle RPM the car would bog terribly on the slightest elevation change, grossly decreasing efficiency. In ther words the consumption vs. RPM curve is not linear. Did BMW hit the sweet spot? Maybe, maybe not. I suppose that depends on many factors, some of which will vary between driver, environment, etc. My guess is that they spend a lot of time on this stuff and probably arrived at the current gearing after much experimentation, data collection, and deliberation.
An 8 speed DCT might be part of the answer - those will probably be here sometime later this decade if DCT tech is not killed off in favor of a traditional planetary gearbox.